In a liquid crystal display element, in order to make a display property of a liquid crystal screen uniform, it is desirable to reduce nonuniformity of a temperature of a liquid crystal panel as small as possible. The heat generation is caused mainly by heat conduction from a driving power source or a back light. Besides the above heat conduction, a liquid crystal panel also generates a heat by application of a signal voltage.
In a liquid crystal display panel using a nematic liquid crystal (NLC), the heat generation from the liquid crystal panel is too small to be worth considering because a capacitance of a panel is low, a driving voltage is low, i.e. about 5 V, and a driving frequency is low, i.e. about several kHz. For this reason, in this case, the heat which influences the display property is caused mainly by heat conduction from a heat generating source such as a driving power source and a back light. Therefore, in the liquid crystal display device using NLC, in order to suppress the heat conduction from the heat generating source as low as possible, and to maintain a temperature distribution on the liquid crystal panel uniform, the following attempts are made.
For example, Japanese Unexamined Patent Publication No. 4-115286/1992 (Tokukaihei 4-115286) discloses an arrangement that a heat from a heat generating source is agitated by forced convection such as a fan and is released out of a display device. Moreover, Japanese Unexamined Patent Publication No. 3-174134/1991 (Tokukaihei 3-174134) discloses an arrangement that heat conduction from a heat generating source to a liquid crystal panel is cut off by a heat pipe. Furthermore, Japanese Unexamined Patent Publication No. 4-62520/1992 (Tokukaihei 4-62520) discloses an arrangement that heat generation of a back light is absorbed and diffused by a coolant. Moreover, Japanese Unexamined Patent Publications No. 3-50591/1991 and 4-172319/1992 (Tokukaihei 3-50591 and 4-172319) disclose an arrangement that a partial heat conduction is prevented by using a heat diffusing plate which diffuses a heat from a back light.
The above arrangements disclosed in each Publication prevent a heat generated in inner or outer apparatuses such as a back light from being conducted to a liquid crystal panel, and this prevents the display property from becoming nonuniform due to temperature change of the liquid crystal.
The smaller power consumption is, the smaller the calorific value becomes. For this reason, as a technique that suppresses the power consumption of a driving circuit so as to make a calorific value small, two arrangements are known: an arrangement disclosed in Japanese Unexamined Patent Publication No. 6-19422/1994 (Tokukaihei 6-19422) that power supply to a liquid crystal display driver is intermittently suspended, and an arrangement disclosed in Japanese Unexamined Patent Publication No. 7-56537/1995 (Tokukaihei 7-56537) that a useless power on a circuit configuration is limited. These arrangements are mainly used as means for controlling the heat generation of the driving circuit.
However, in the above conventional arrangements, the heat generation of the liquid crystal display element due to the driving of liquid crystal is not considered. In recent years, a liquid crystal display device in which a thickness of a liquid crystal layer of a liquid crystal display element (cell thickness) is thin, such as a liquid crystal display element using surface stabilized ferroelectric liquid crystal (SSFLC) or the like is known. In such a liquid crystal display element, a temperature distribution on a panel becomes nonuniform because the liquid crystal display element functions as a heat generating source at the time of driving the liquid crystal, and thus such a problem that a uniform display state cannot be obtained arises easily.
A calorific value of the liquid crystal display element depends upon charge and discharge electric currents generated in picture elements. The calorific value becomes larger in a portion of the liquid crystal display element which is closer to an input terminal of a signal line voltage, and becomes smaller in a portion of the liquid crystal display element which is farther from the input terminal of the signal line voltage. This is because each picture element on the signal line electrode of the liquid crystal display element can be represented as a capacitor-resistor (CR) circuit, a voltage waveform to be applied to each picture element is close to an applied voltage waveform near the input terminal, but the waveform becomes less steep as the portion is farther from the input terminal, and thus the charge and discharge electric currents become small because a rising of the voltage gets slow. As mentioned above, when the distribution of the calorific value on the liquid crystal display element becomes nonuniform, a difference in temperature on the panel occurs. The difference in temperature on the panel produces a problem that the driving property of the liquid crystal is made nonuniform, and thus the display property of an image is deteriorated.
In the conventional liquid crystal display element using NLC, the influence of the heat generation on the liquid crystal display element is an order of 1/100 of the liquid crystal display element using SSFLC. This is because a capacitance, namely, a capacitive load of the panel is small since a cell thickness of NLC is thicker than SSFLC, and the driving frequency of the liquid crystal is not more than several kHz, and the driving voltage during a non-selecting period is low, i.e. not more than about 5 V.
On the contrary, the cell thickness of an SSFLC panel, for example, is 1-2 .mu.m, namely, it has a thinner thickness than the several .mu.m cell thickness of the NLC panel, and a capacitance of the SSFLC panel is larger than a general NLC panel. In other words, the capacitive load of the SSFLC panel is larger than the NLC panel. Moreover, in the SSFLC panel, the driving frequency is an order of dozens to hundred kHz, and the driving voltage of about 5 to 15 V is required for the non-selecting period. According to these points, in the liquid crystal display element having a thin cell thickness, even if no heat conduction from a back light or a voltage source occurs, since the calorific value of the liquid crystal display element is comparatively large, there is a difference in temperature of several 0.degree. C. depending on a portion of the display screen. This difference in temperature causes variations of the driving property of the liquid crystal in the display screen, and thus there arises a problem of nonuniform display.